Eraser. As far as I am concerned, there are only two reliable ways to erase an old hard disk. One involves a big claw hammer. The other is this little open-source software gem.
Eraser works not just on hard disks but also on rewritable CDs and DVDs, USB drives, flash memory, and any other storage medium. Just how thorough a job it does is up to you; if used properly, Eraser will ensure that nobody can recover your deleted data, even if they resort to complex (and absurdly expensive) hardware-based recovery methods.
Other handy features include a built-in scheduling tool, the ability to create customized overwriting routines, and options to erase only the files (including stuff like Windows alternate data streams) or free space on a drive.
If you like Eraser, by the way, you might also want to check out a related application, known as Darik's Boot and Nuke (DBAN): a bootable floppy that will automatically detect and erase every storage device attached to a PC (although if you're like me and have a habit of leaving unlabeled disks sitting around, maybe you should just steer clear of this one).
Axcrypt. Of course, the best way to keep really important data secure is to ensure that it only exists on your PC in encrypted form. Axcrypt, like Eraser, provides excellent protection when used properly; its encryption algorithms are, for example, far more robust than the limited password-protection feature built into the Windows Compressed Folder tool. Axcrypt works on any amount of data, ranging from individual files and folders to a complete disk; and it supports quick and easy encryption/decryption via Windows Explorer context menus.
There is actually a lot more to Axcrypt, but one of the great things about this tool is the fact that you can start using it with very little effort: Installing it is a snap, and most users will find that the default configuration settings work just fine. Also, keep in mind that Axcrypt gets a lot of care and attention from a large, active open-source developer community; with so many trained eyes scanning the source code, there is very little risk that a hidden "back door" or other security exploit might compromise your personal data.